High transmittance glass sheet and method of manufacturing the same

ABSTRACT

A high transmittance glass sheet is provided that is formed of a soda-lime-silica glass composition containing, expressed in wt. %, less than 0.020% of total iron oxide in terms of Fe 2 O 3  and 0.006 to 2.0% of zinc oxide. The glass sheet allows the formation of nickel sulfide particles to be suppressed by the addition of a zinc compound to a glass raw material.

This is a divisional of application Ser. No. 10/236,397, filed Sep. 5,2002 now U.S. Pat. No. 6,831,030.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a high transmittance glass sheet ofsoda-lime-silica glass manufactured mainly by a float process. Morespecifically, this invention relates to a high transmittance glass sheetthat allows the formation of nickel sulfide (NiS) in a process ofmelting a glass raw material to be suppressed effectively.

2. Related Background Art

In methods of manufacturing a soda-lime-silica glass sheet such as afloat process and a roll out process, the following problem may arise.That is, in a process of melting raw materials in a furnace at a hightemperature near 1,500° C., metal particles of stainless steel or thelike containing nickel (Ni) may be mixed into the raw materials. Themetal particles may react with sulfur (S) in salt cake (Na₂SO₄) includedin the raw materials. As a result of this reaction, nickel sulfide (NiS)is formed as minute foreign matter in glass products. NiS particles arepresent at a minimal rate of about one per a little over 10 tons ofglass products, and are of an extremely minute spherical form having adiameter as small as about 0.3 mm or less. Therefore, it is difficult todetect NiS particles on production lines.

Some soda-lime-silica glass sheets are tempered to be used forbuildings, vehicles, cover glass plates for solar cell panels, solarwater heaters or the like. In a tempering process, a glass sheet isheated to a temperature near the softening point (about 600° C.) of theglass sheet. Then, the glass sheet is quenched so that compressivestress layers are generated in surfaces of the glass sheet.

When NiS is contained in a tempered glass sheet, NiS is present in an αphase that is stable at about 350° C. or higher, and undergoes phasetransition with the lapse of time to a β phase that is more stable atroom temperature. This phase transition causes NiS particles to expandin volume. As a result of this, micro cracks may appear in the vicinityof the NiS particles. Inside the tempered glass sheet, a tensile stresslayer exists, having a thickness of about two-thirds that of the glasssheet. When cracks appear in the tensile stress layer, the cracks runrapidly to cause spontaneous fracture of the tempered glass sheet.

To prevent such spontaneous fracture of a tempered glass sheet,so-called soaking has been known. In this method, tempered glass sheetsare heated to 300° C. or lower in a furnace (soaking furnace). Then, thetempered glass sheets are maintained in the furnace for a predeterminedtime, so that NiS undergoes phase transition from an α phase to a βphase. This forces the tempered glass into breakage. In this manner,defective glass products containing NiS are eliminated.

However, operations such as the soaking in which heat treatment ismainly performed cost considerable energy and time, thereby causing anincrease in manufacturing cost. This also is a serious hindrance toshortening of delivery times and an improvement in productivity.Further, defective products are eliminated in the soaking, therebycausing a decrease in product yield.

JP 9(1997)-169537 A discloses a method of manufacturing asoda-lime-silica glass in which 0.01 to 0.15 wt. % of a zinc compoundsuch as zinc nitrate and zinc sulfate is added to raw materials, therebyallowing the formation of NiS to be suppressed.

Meanwhile, there has been a growing demand that a high transmittanceglass sheet, more specifically, a glass sheet having a light color and ahigh transmittance be used for an interior glass, a showcase, a displaycase, a high transmittance non-colored window glass, a hightransmittance non-colored mirror, a glass substrate for a solar cellpanel, a cover glass plate for a solar cell panel, a solar water heater,a material for a high solar-heat transmittance window glass, and a flatdisplay substrate glass such as a front panel or the like. However, nohigh transmittance glass sheet has been known so far that is suitablefor industrial mass production.

SUMMARY OF THE INVENTION

A high transmittance glass sheet according to the present invention isformed of a soda-lime-silica glass composition containing, expressed inwt. %, less than 0.020% of total iron oxide and 0.006 to 2.0% of zincoxide. In this specification, total iron oxide denotes an amount of ironoxide in terms of Fe₂O₃. All of the iron in the composition is countedas Fe₂O₃, even if it exists as FeO.

The soda-lime-silica glass composition constituting the hightransmittance glass sheet according to the present invention containsless than 0.020 wt. % (less than 200 ppm) of total iron oxide. Bymaintaining the content of total iron oxide at a low level as describedabove, it is made easier to obtain a high transmittance glass sheethaving, on a 4.0 mm thickness basis, a solar radiation transmittance of87.5% or higher. Preferably, total iron oxide is contained in an amountof not less than 0.005 wt. % as will be described later.

For the effective suppression of NiS formation in a soda-lime-silicaglass composition containing less than 200 ppm of total iron oxide, zincoxide should be contained, in terms of ZnO, in an amount of not lessthan 0.006 wt. % (not less than 60 ppm). The addition of zinc oxide doesnot cause an increase in light absorption in the visible light region.It has been found to be desirable for the suppression of NiS formationthat the content of zinc oxide be increased in inverse proportion to thecontent of total iron oxide. When the content of total iron oxide has avalue near 200 ppm, it is required that ZnO be contained in an amount ofnot less than 60 ppm. When the content of total iron oxide is 50 ppm,preferably, ZnO is contained in an amount of not less than 180 ppm. Morepreferably, when the content of total iron oxide has a value near 200ppm, ZnO is contained in an amount of not less than 100 ppm, and whenthe content of total iron oxide is 50 ppm, ZnO is contained in an amountof not less than 300 ppm.

In manufacturing a high transmittance glass, in order to prevent ZnOfrom volatilizing during melting to damage a furnace, ZnO should becontained in an amount of not more than 2.0 wt. % (not more than 20,000ppm). In the case where a float bath is used for forming a glass sheet,in order to prevent ZnO that has volatilized and condensed in the floatbath from dropping onto a glass ribbon to form a defect, ZnO is useddesirably in an amount of not more than 5,000 ppm, and more desirably inan amount of not more than 1,000 ppm.

This problem, which is caused by dropping of a condensed material thathas volatilized, does not occur in the case where a glass sheet ismanufactured, instead of using the float bath, for example, by a rollout process in which molten glass is rolled using a roller with anuneven (a predetermined pattern) or an even surface, and by a process inwhich molten glass that has been allowed to pass through a slit oroverflow from a melting tub is drawn.

Thus, as shown in FIG. 1, where an x-coordinate axis indicates thecontent of the total iron oxide expressed in ppm and a y-coordinate axisindicates the content of the zinc oxide expressed in ppm, the glasscomposition has contents of the total iron oxide and the zinc oxidewhose values fall preferably within a range defined by a square ABCDformed by connecting Point A (200, 60), Point B (200, 20,000), Point C(50, 20,000), and Point D (50, 180) in this order, more preferablywithin a range defined by a square A′BCD′ formed by connecting Point A′(200, 100), Point B (200, 20,000), Point C (50, 20,000), and Point D′(50, 300) in this order, and most preferably within a range defined by asquare A′B′C′D′ formed by connecting Point A′ (200, 100), Point B′ (200,5,000), Point C′ (50, 5,000), and Point D′ (50, 300) in this order.

The present invention also provides a method of suppressing formation ofnickel sulfide in a high transmittance glass sheet having a solarradiation transmittance of 87.5% or higher and/or a visible lighttransmittance of 90.0% or higher on a basis of a 4.0 mm thick glasssheet. In the method, a glass raw material is prepared so that a contentof total iron oxide in terms of Fe₂O₃ is less than 0.020 wt % and acontent of zinc oxide is 0.006 to 2.0 wt. %, and the glass raw materialis melted.

The content of zinc oxide required to suppress the formation of nickelsulfide particles in a glass composition increases as the content oftotal iron oxide is decreased when the content of the total iron oxidein the glass is in the range of 0.006 to 0.060 wt. %. Since zinc oxidematerials are costly compared with other raw materials, it would be costeffective to use zinc oxide in the least possible amount required tosuppress the formation of nickel sulfide particles. Therefore, inmanufacturing soda-lime glasses successively, when the content of totaliron oxide in a glass composition is decreased over time, preferably,the content of zinc in the glass composition is increased accordinglywithin the range of 0.006 to 0.50 wt. % (60 to 5,000 ppm). Conversely,when the content of the total iron oxide in the glass composition isincreased over time, preferably, the content of zinc in the glasscomposition is decreased accordingly in the above range.

Examples of zinc compounds for zinc oxide (ZnO) that should be added toa raw material include an inorganic zinc compound such as zinc nitrate(Zn(NO₃)₂.6H₂O), zinc sulfate (ZnSO₄.7H₂O), a zinc halide (e.g. zincfluoride (ZnF₂.4H₂O), zinc bromide (ZnBr₂), zinc chloride (ZnCl₂) andzinc iodide (ZnI₂)) and zinc phosphate (Zn₃(PO₄)₂.4H₂O); and an organiczinc compound such as zinc benzoate (Zn(C₆H₅CO₂)₂) and zinc acetate(Zn(CH₃CO₂)₂.2H₂O). Although these zinc compounds have substantially thesame effects, it is most preferable to use at least one selected fromzinc nitrate and zinc sulfate from the viewpoint of cost effectivenessor the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a preferred relationship between the contentof total iron oxide and the content of zinc oxide in a glass compositionaccording to the present invention.

FIG. 2 is a graph showing a relationship between the respective contentsof T-Fe₂O₃ and CeO₂ and a fluorescence intensity ratio.

FIG. 3 is a graph showing a relationship between the content of Fe₂O₃and a number of NiS particles formed in a soda-lime-silica glass.

FIG. 4 is a graph showing a relationship between an amount of Ni addedand the content of ZnO required to reduce the number of NiS particles tohalf in a soda-lime-silica glass.

DETAILED DESCRIPTION OF THE INVENTION

The high transmittance glass sheet according to the present invention isformed of a glass composition containing total iron oxide and zinc oxideas described above. In the following description, the glass compositionwill be explained in greater detail.

Preferably, the high transmittance glass sheet according to the presentinvention has the following features. That is, the glass sheet is formedof a soda-lime-silica glass composition that contains in addition to thezinc oxide, expressed in wt. %, 0.005 to less than 0.020% of total ironoxide (hereinafter, referred to as T-Fe₂O₃) in terms of Fe₂O₃, less than0.008% of FeO, and 0 to 0.25% of cerium oxide, and has a ratio(hereinafter, referred to as a FeO ratio) of the content of FeO in termsof Fe₂O₃ to the content of T-Fe₂O₃ of lower than 40%. When measurementsare made on a 4.0 mm thickness basis, the high transmittance glass sheetpreferably has a solar radiation transmittance of 87.5% or higher, avisible light transmittance as determined using the CIE standardilluminant C of 90.0% or higher, a dominant wavelength as determinedusing the illuminant C of 540 to 580 nm, and an excitation purity asdetermined using the illuminant C of 0.35% or lower. Here, the content(wt. %) of the zinc oxide is expressed by a value of an amount of thezinc oxide added with respect to a total amount of 100 wt. % of theother components.

More preferably, the high transmittance glass sheet has the followingfeatures. That is, the glass sheet is formed of a composition that issubstantially free from cerium oxide (the content of CeO₂ is less than0.005 wt. %) and has a FeO ratio of equal to or higher than 22% to lowerthan 40%. In this case, when a measurement is made on a 4.0 mm thicknessbasis, the high transmittance glass sheet has an excitation purity asdetermined using the illuminant C of 0.25% or lower. This compositionallows a high transmittance and extremely light colored glass sheet.

Furthermore, a high transmittance glass sheet that is formed of a glasscomposition containing 0 to 0.005 wt. % of cerium oxide, not more than0.03 wt. % of manganese oxide, and not more than 0.01 wt. % of vanadiumoxide can achieve the following. That is, when the high transmittanceglass sheet is exposed to ultraviolet radiation at a wavelength of notmore than 400 nm, for example, ultraviolet irradiation according to thelight stability test specified in the Japanese Industrial Standards,R3212, on a 4.0 mm thickness basis, the transmittance (in thenear-infrared region) at a wavelength of 1,000 nm can be improved by notless than 0.1%, and in some cases, by not less than 0.3% with respect tothat of the glass sheet before being exposed to the ultravioletradiation. Furthermore, after the ultraviolet irradiation, the solarradiation transmittance and the visible light transmittance of the hightransmittance glass sheet also can be increased to 90.0% or higher and90.5% or higher, respectively. Although not entirely clarified,conceivably, lowering of the FeO ratio contributes to these improvementsin the transmittance in the near-infrared region. For example, even whena glass sheet has a FeO ratio of 22% or higher, ultraviolet irradiationallows the FeO ratio to be lowered by 3 to 5%, so that the content ofFeO can be reduced to lower than 22%.

Furthermore, it also is more preferable that the high transmittanceglass sheet has the following features. That is, the high transmittanceglass sheet is formed of a composition that contains, expressed in wt.%, 0.02 to 0.25% of cerium oxide, and has a FeO ratio of lower than 22%.When measurements are made on a 4.0 mm thickness basis, the hightransmittance glass sheet has a solar radiation transmittance of 90.0%or higher and a visible light transmittance as determined using theilluminant C of 90.5% or higher. This allows a high transmittance glasssheet to be obtained that exhibits a high transmittance particularly ina region ranging from the visible region to the near-infrared region.

Furthermore, particularly for the efficient conversion of ultravioletlight into visible light, a high transmittance glass sheet is preferredthat contains, expressed in wt. %, 0.025 to 0.20% of cerium oxide, andhas a ratio of a fluorescence intensity at a wavelength of 395 nm to afluorescence intensity at a wavelength of 600 nm (f(395 nm)/f(600 nm),hereinafter, referred to also as a fluorescence intensity ratio) of 10or higher when subjected to ultraviolet irradiation at a wavelength of335 nm. Furthermore, a high transmittance glass sheet is desired thatcontains 0.03 to 0.15 wt. % of cerium oxide and has a fluorescenceintensity ratio of 15 or higher. Moreover, a high transmittance glasssheet is desired most that contains 0.05 to 0.12 wt. % of cerium oxideand has a fluorescence intensity ratio of 25 or higher, since the glasssheet allows most efficient conversion of ultraviolet light into visiblelight.

Preferably, the above-mentioned soda-lime-silica glass compositionaccording to the present invention contains, in addition to the ironoxide, the zinc oxide and the cerium oxide that are described above, ascomponents constituting a base glass composition, expressed in wt. %, 65to 80% of SiO₂, 0 to 5% of Al₂O₃, 0 to 7% of MgO, 5 to 15% of CaO, wherea total content of MgO and CaO is more than 7% and not more than 17%, 10to 18% of Na₂O, 0 to 5% of K₂O, where a total content of Na₂O and K₂O is10 to 20%, and 0.05 to 0.3% of SO₃. The content of the above-mentionedzinc oxide is expressed by an amount of the zinc oxide added withrespect to a total amount of 100% of the above components constitutingthe base glass composition.

Furthermore, more preferably, the total content of MgO and CaO (MgO+CaO)is 10 to 17 wt. %, and the content of SO₃ is 0.08 to 0.15 wt. %.Moreover, it is desirable that the content of MgO be 0.5 to 7 wt. %since this allows meltability and formability to be improved.Furthermore, it is desirable that the content of Al₂O₃ be 0.5 to 5 wt. %since this allows water resistance to be improved.

Hereinafter, the composition of the high transmittance glass sheetaccording to the present invention will be described in terms of thecomponents other than the zinc oxide described earlier. In the followingdescription, the respective contents of the components are expressed inwt. %.

In glass, iron oxide is present in forms of Fe₂O₃ and FeO. Fe₂O₃ servesto enhance an ultraviolet-absorbing ability, and FeO serves to enhance aheat-absorbing ability. In order to attain a high transmittance asdesired, preferably, the content of T-Fe₂O₃ (a total content of Fe₂O₃and FeO in terms of Fe₂O₃) is less than 0.020%, and preferably, thecontent of FeO is less than 0.008%, and the FeO ratio is lower than 40%.When the contents of T-Fe₂O₃ and FeO and the FeO ratio reach and becomegreater than their respective upper limits, the visible lighttransmittance becomes too low, and the glass takes more on a blue toneof FeO.

When the content of T-Fe₂O₃ is less than 0.005%, it is necessary to usehigh purity materials having low iron contents. This leads to asubstantial cost increase. Thus, preferably, T-Fe₂O₃ is contained in anamount of not less than 0.005%.

When used for a glass substrate and a cover glass plate for a solar cellpanel using amorphous silicon, a glass sheet preferably has a hightransmittance with respect to light having a wavelength in the vicinityof 500 to 600 nm and exhibits moderate solar radiation absorption. Inthis case, preferably, when the content of T-Fe₂O₃ is in theabove-mentioned range, the content of FeO is more than 0.003% and lessthan 0.008%, and the FeO ratio is equal to or higher than 22% and lowerthan 40%.

When used for a glass substrate and a cover glass plate for a solar cellpanel using crystalline silicon, preferably, a glass sheet has a hightransmittance with respect to light having a wavelength in the vicinityof 1,000 nm. In this case, preferably, when the content of T-Fe₂O₃ is inthe above-mentioned range, the content of FeO is less than 0.004%, andthe FeO ratio is lower than 22%.

Cerium oxide (CeO₂) is effective in regulating the content of FeO andthe FeO ratio. Particularly, in order to attain a low FeO content and alow FeO ratio required when a high transmittance at a wavelength in thevicinity of 1,000 nm is desired, preferably, CeO₂ is added in an amountof 0.02 to 0.25%.

Furthermore, with respect to glasses containing 0.005 to 0.08 wt. % ofT-Fe₂O₃ and 0 to 0.20 wt. % of CeO₂, a relationship between the contentof CeO₂ and a fluorescence property is shown in FIG. 2. As shown in FIG.2, it was found that ultraviolet light was absorbed and converted tovisible light most effectively when the content of CeO₂ was in a givenrange. That is, it was found that a high transmittance glass sheet couldbe obtained that contained less than 0.06% of T-Fe₂O₃ and 0.025 to 0.20%of CeO₂, thereby achieving a fluorescence intensity ratio of 10 orhigher, a fluorescence intensity ratio of 15 or higher when the contentof CeO₂ was 0.03 to 0.15%, and a fluorescence intensity ratio of 25 orhigher when the content of CeO₂ was 0.05 to 0.12%.

The high transmittance glass sheet described above is suitable for usefor an interior material, a glass for a showcase or the likeparticularly because the glass sheet takes on a fluorescent color withgradations when ultraviolet light is incident on an edge surface of theglass sheet from a cross sectional direction.

Furthermore, when used for a substrate and a cover glass plate for asolar cell panel or the like, the above-mentioned high transmittanceglass sheet is used most suitably since the glass sheet allows energy inthe ultraviolet region that hardly contributes to power generation to beconverted into light in the visible region, thereby allowing the powergeneration efficiency to be enhanced.

SiO₂ is a main component forming a skeleton of the glass. When thecontent of SiO₂ is less than 65%, the durability of the glass isdecreased, and when the content of SiO₂ is more than 80%, melting of theglass is hindered.

Although not an indispensable component, Al₂O₃ serves to improve thedurability and the water resistance of the glass. When the content ofAl₂O₃ is increased, melting of the glass is hindered. Thus, the contentof Al₂O₃ should be 0 to 5%. In order to improve the durability and thewater resistance, preferably, the content of Al₂O₃ is not less than0.5%. In order not to impair the meltability of the glass, preferably,the content of Al₂O₃ is not more than 2.5%. More preferably, the contentof Al₂O₃ is in the range of 1.0 to 2.5%.

Both MgO and CaO serve to improve the durability of the glass andregulate the liquidus temperature and the viscosity of the glass in aforming process. Although not an indispensable component, MgO allows alow liquidus temperature to be maintained when contained in a moderateamount. Thus, the content of MgO is preferably more than 0.5%, and morepreferably not less than 2%. When the content of MgO exceeds 7%, theliquidus temperature is increased excessively. On the other hand, whenthe content of CaO is less than 5%, the meltability is degraded.Further, when the content of CaO exceeds 15%, the liquidus temperatureis increased. Thus, more preferably, the content of CaO is not more than13%. When a total content of MgO and CaO is not more than 7%, thedurability of the glass is decreased. Conversely, when the total contentexceeds 17%, the liquidus temperature is increased. Thus, morepreferably, the total content is not more than 15%. In the case wherethe total content of MgO and CaO is as small as, for example, less than10%, it is required that the content of Na₂O be increased so that thedegradation of the meltability and an increase in viscosity of a meltare compensated. This leads to a cost increase and a decrease inchemical durability of the glass. Thus, more desirably, the totalcontent of MgO and CaO is not less than 10%.

Both Na₂O and K₂O serve to accelerate melting of the glass. When thecontent of Na₂O is less than 10% or when a total content of Na₂O and K₂Ois less than 10%, only a poor effect of accelerating glass melting canbe obtained. It is not preferable that the content of Na₂O exceeds 18%or the total content of Na₂O and K₂O exceeds 20% since this results in adecrease in the durability of the glass. In applications where waterresistance is required particularly, the content of Na₂O is preferablynot more than 15%, and more desirably not more than 14.5%. Since amaterial cost of K₂O is high compared with Na₂O, K₂O is not anindispensable component. Even when K₂O is used, it is not preferablethat the content of K₂O exceeds 5%.

SO₃ serves to accelerate clarification of the glass. When the content ofSO₃ is less than 0.05%, a sufficient clarifying effect cannot beattained by a regular melting method. Thus, desirably, the content ofSO₃ is more than 0.1%. Conversely, when the content of SO₃ exceeds 0.3%,SO₂ produced as a result of decomposition of SO₃ remains in the glass inthe form of a bubble, and SO₃ dissolved in the glass becomes more likelyto produce bubbles by reboiling.

Although not an indispensable component, TiO₂ can be added in a properamount for the purposes of enhancing an ultraviolet-absorbing ability orthe like as long as the amount is in the range that allows the opticalproperties that are the intended properties of the present invention notto be impaired. When an excessive amount of TiO₂ is contained, the glassbecomes more likely to be yellowish, and the transmittance at awavelength in the vicinity of 500 to 600 nm is lowered. Thus, desirably,the content of TiO₂ is limited to a low level in the range of less than0.2%.

Furthermore, even when fluorine, boron oxide, barium oxide, andstrontium oxide are contained, the effect of the present invention isnot impaired. However, these components create adverse impacts such as acost increase, shortening a furnace life, release of harmful substancesinto the air or the like. Thus, the glass composition should besubstantially free from these components.

As a component to be added as an oxidizing agent to a glass having theabove-mentioned composition, preferably, cerium oxide in an amount inthe range defined in the above description is used, in view of theeffect of cerium oxide and an ultraviolet-absorbing effect as anotherparticular effect of cerium oxide. However, an oxidizing agent otherthan cerium oxide, for example, manganese oxide may be added in anamount in the range of not more than 1% in combination with cerium oxideor as an only oxidizing agent.

Furthermore, SnO₂ may be added as a reducing agent in an amount in therange of not more than 1%. Moreover, as in a general case, in additionto the iron oxide, the cerium oxide and the manganese oxide that aredescribed above, concurrently with the addition of these components, atleast one selected from the group consisting of Se, CoO, Cr₂O₃, NiO,V₂O₅, MoO₃ or the like may be added as a coloring agent, in an amount inthe range that allows the high transmittance that is the intendedproperty of the present invention not to be impaired. However, when thecoloring agent is added in an excessive amount, a color tone isintensified and the visible light transmittance is lowered. Thus,desirably, these compounds are not added practically. For example,desirably, the content of V₂O₅ is not more than 0.01 wt. %.

The effect of the high transmittance glass according to the presentinvention can be attained effectively when the high transmittance glassis subjected to quenching (tempering).

The high transmittance glass sheet according to the present invention ishighly demanded particularly in use for a solar cell panel. When theglass sheet is used for a solar cell panel, an anti-reflecting film anda conductive film can be formed on the glass sheet. Even when thesefilms are formed on the glass sheet, the glass properties are notaffected. Further, regardless of whether these films are formed, theglass sheet can be subjected to processing involving heating such astempering and bending. Generally, in rapidly cooling for tempering, thehigh transmittance glass sheet is heated to a temperature near thesoftening point of the glass sheet, and then cooled rapidly by beingbrought into contact with cold air or other fluids.

The high transmittance glass sheet according to the present inventiongenerally has a thickness of 0.3 mm to 30 mm and is suitable for use foran interior glass, a showcase, a display case, a high transmittancenon-colored window glass, a high transmittance non-colored mirror, aglass substrate for a solar cell panel, a cover glass plate for a solarcell panel, a solar water heater, a high solar-heat transmittance windowglass, a window glass for a microwave oven, or a flat display substrateglass such as a front panel or the like.

EXAMPLE

With respect to soda-lime-silica glass in which Ni metal is containedintentionally, a relationship between the content of total iron oxide(in terms of Fe₂O₃) and the degree to which NiS is likely to be formedwas examined. As can be seen from the results of the examination shownin FIG. 3, it was observed that NiS became more likely to be formed asthe content of the total iron oxide was decreased from 0.20 wt. %, andin particular, the number of NiS particles formed increased steeply whenthe content of the total iron oxide was not more than 0.060 wt. %. Table1 shows the respective values of the content of the total iron oxide, anamount of the Ni metal added, the number of the NiS particles formed,and the maximum diameter of the NiS particles that were used to obtainthe results shown in FIG. 3. These results were obtained by using acrucible having a capacity of 250 cm³. Also in actual melting andforming operations of soda-lime-silica glass using a tank-type meltingfurnace, it was confirmed that a ratio of defects in a tempered glasssheet caused by soaking increased as the content of iron oxide in theglass was decreased from 0.20 wt. %.

TABLE 1 Sample 1 Sample 2 Sample 3 Content of total 0.018 0.050 0.200iron oxide (wt. %) Content of Ni* 700 700 700 (ppm) NiS (no. ofparticles/ 323 113 50 100 g of glass) Maximum diameter 150 120 120 ofNiS particle (μm) *Ni particles contained have a diameter of 149 μm.

Two types of raw materials varying in Fe₂O₃ content were prepared bymixing reagent chemicals or equivalents of SiO₂, Al₂O₃, MgO, CaCO₃,Na₂CO₃, K₂CO₃, TiO₂, Na₂SO₄, Fe, and carbon (C). In Table 2, CompositionNo. 1 represents soda-lime-silica glass having a Fe₂O₃ content of lessthan 0.02 wt. %, and Composition No. 2 represents soda-lime-silica glasshaving a Fe₂O₃ content of 0.05 wt. %. A relationship between therespective amounts of Na₂SO₄ and carbon that were used and an amount ofresultant SO₃ was determined beforehand. In each composition, based onthe relationship, an amount of Na₂SO₄ in terms of Na₂O to be added wasset to 0.74 wt. % so that the amount of resultant SO₃ shown in Table 2was attained. The amount of Na₂O was regulated so that the amount shownin Table 2 was attained by using Na₂CO₃. In Table 2, the respectivecontents are expressed in wt. %.

TABLE 2 Composition No. 1 Composition No. 2 SiO₂ 71.9 71.8 Al₂O₃ 1.9 1.9MgO 3.9 3.9 CaO 8.2 8.2 Na₂O 13.6 13.6 K₂O 0.3 0.3 TiO₂ 0.030 0.030 CeO₂0 0 MnO₂ 0 0 SO₃ 0.200 0.200 T—Fe₂O₃ 0.018 0.050 FeO in T—Fe₂O₃ 0.0050.015 Total 100.0 100.0

Glass batch materials of Samples 4 to 47 were prepared in the followingmanner. That is, with respect to each of these two types of rawmaterials, a powder of Ni metal having a particle diameter of 149 μm anda powder of zinc nitrate (Zn(NO₃)₂.6H₂O) or zinc sulfate (ZnSO₄.7H₂O)are added in the respective amounts shown in Tables 3 and 4. In thetables, A and B in the column titled “additive” represent zinc sulfate(ZnSO₄.7H₂O) and zinc nitrate (Zn(NO₃)₂.6H₂O), respectively.

With respect to each sample, a batch of these materials was put in analumina crucible having a capacity of 250 cc and preheated at atemperature of 600° C. for 30 minutes. Then, the batch was inserted inan electric furnace maintained at a temperature of 1,370° C., and thetemperature of the electric furnace was raised to 1,400° C. in 10minutes. After being maintained at this temperature for 2.2 hours, thebatch was taken out of the electric furnace and cast out to be annealedto room temperature, so that a glass sheet was obtained.

With respect to each glass obtained, the number of NiS particles wasmeasured using a stereomicroscope. The results of the measurements areshown in Tables 3 and 4.

TABLE 3 NiS Amount Amount of (no. of of Ni additive in particles/Composition added terms of 100 g of No. (ppm) Additive* ZnO (ppm) glass)Sample 4 1 350 A 0 300 Sample 5 1 350 A 100 270 Sample 6 1 350 A 200 290Sample 7 1 350 A 300 220 Sample 8 1 350 A 400 230 Sample 9 1 350 A 1,000110 Sample 10 1 140 A 0 52 Sample 11 1 140 A 200 24 Sample 12 1 35 A 0 4Sample 13 1 35 A 200 3 Sample 14 1 35 A 300 2 Sample 15 1 35 A 400 0Sample 16 1 140 B 0 52 Sample 17 1 140 B 300 9 Sample 18 2 350 B 0 57Sample 19 2 350 B 27 52 Sample 20 2 350 B 68 46 Sample 21 2 350 B 103 40Sample 22 2 350 B 205 33 Sample 23 2 350 B 410 4 Sample 24 2 175 B 0 27Sample 25 2 175 B 27 19 *Additive A represents zinc sulfate (ZnSO₄.7H₂O)Additive B represents zinc nitrate (Zn(NO₃)₂.6H₂O)

TABLE 4 NiS Amount Amount of (no. of of Ni additive in particles/Composition added terms of 100 g of No. (ppm) Additive* ZnO (ppm) glass)Sample 26 2 175 B 68 17 Sample 27 2 175 B 103 14 Sample 28 2 175 B 20510 Sample 29 2 87.5 B 0 13 Sample 30 2 87.5 B 27 8 Sample 31 2 87.5 B 682 Sample 32 2 87.5 B 103 1 Sample 33 2 87.5 B 205 0 Sample 34 2 350 A 057 Sample 35 2 350 A 50 54 Sample 36 2 350 A 126 50 Sample 37 2 350 A189 46 Sample 38 2 350 A 378 35 Sample 39 2 350 A 756 14 Sample 40 2 175A 0 27 Sample 41 2 175 A 50 20 Sample 42 2 175 A 126 10 Sample 43 2 175A 189 2 Sample 44 2 175 A 378 2 Sample 45 2 87.5 A 0 15 Sample 46 2 87.5A 50 9 Sample 47 2 87.5 A 126 0

It can be seen from Tables 3 and 4 that in each of the glassescontaining 0.050 wt. % of T-Fe₂O₃ (Composition No. 2), by adding a traceamount of zinc nitrate (Zn(NO₃)₂.6H₂O) or zinc sulfate (ZnSO₄.7H₂O) tothe glass materials, a considerable effect of suppressing the formationof NiS in a glass product can be obtained. On the other hand, it can beseen from the tables that, in each of the glasses containing less than0.02 wt. % of T-Fe₂O₃ (Composition No. 1), an effect of preventing theformation of NiS cannot be obtained when the amount of zinc nitrate orzinc sulfate added is small, and can be obtained by increasing theamount of the zinc nitrate or the zinc sulfate to be added.

Based on the results shown in Tables 3 and 4, with respect to each ofglasses having the respective Fe₂O₃ contents, the rate of an amount ofan additive in terms of ZnO at which the amount of NiS formed wasreduced to half was determined. The results are plotted in FIG. 4. As isapparent from FIG. 4, compared with the glass containing 0.050 wt. % ofT-Fe₂O₃, in the glass containing 0.018 wt. % of T-Fe₂O₃, in order toreduce the amount of NiS formed to half, it is required that zincnitrate or zinc sulfate in terms of ZnO be used in a two-to four-foldamount, namely, of about not less than 100 ppm.

Examples 1 to 18

Glass batch materials having compositions shown in Tables 5 to 7, inwhich the respective contents are expressed in terms of oxide and in wt.%, were prepared using low-iron silica sand, alumina, limestone,dolomite, soda ash, salt cake, magnesium oxide, cerium oxide, manganesedioxide, zinc sulfate (ZnSO₄.7H₂O), and a carbon-based reducing agent.Each batch of these materials was heated in an electric furnace to atemperature of 1,450° C. to be melted. After four hours of melting, thebatch was poured onto a stainless steel plate and annealed to roomtemperature, so that a glass sheet having a thickness of about 10 mm wasobtained. In the tables, the values of concentration are expressed inwt. %.

After that, the surface of the glass sheet was ground so that a glasssheet sample having a thickness of 4.0 mm was obtained. With respect toeach sample thus obtained, measurements were performed using theilluminant C for optical properties that are a visible lighttransmittance, a dominant wavelength, an excitation purity, a solarradiation transmittance, and a fluorescence intensity ratio. Thefluorescence intensity ratio was determined in the following manner.That is, each of the samples described above was subjected toultraviolet irradiation at a wavelength of 335 nm, and a fluorescenceintensity was determined at the respective wavelengths. Then, acalculation was performed using the formula, fluorescence intensityratio=(fluorescence intensity at 395 nm/fluorescence intensity at 600nm) as an index of the fluorescence intensity. Further, water resistancewas evaluated by determining an elution amount of Na₂O (mg) according toJIS 3502. The respective values of the optical properties and the waterresistance of each sample as the results of the measurements are shownin Tables 5 to 7.

In the same manner as in the above cases of Examples 1 to 18, 18 typesof glass sheets (ZnO-added and Ni-added samples) that were about 10 mmin thickness were obtained. However, for each of these glass sheets, inpreparation of a raw material, a powder of Ni metal having a particlediameter of 149 μm further was added in an amount of 150 ppm withrespect to a total amount of the raw material (in terms of oxide).Further, in the same manner as in the above cases of the Examples 1 to18, 18 types of glass sheets (ZnO-free and Ni-added samples) that wereabout 10 mm in thickness were obtained. However, for each of these glasssheets, in preparation of a raw material, zinc sulfate was not added,and a powder of Ni metal having a particle diameter of 149 μm was addedin an amount of 150 ppm with respect to a total amount of the rawmaterial (in terms of oxide).

With respect to these two sets of samples, measurements were performedfor the number of NiS particles using a stereomicroscope. As the resultsof the measurements, in the samples to which ZnO was not added and Niwas added, 30 to 50 NiS particles per 100 g of glass were observed,while in the samples to which ZnO was added and Ni was added, 0 to 10NiS particles per 100 g of glass were observed.

TABLE 5 1 2 3 4 5 6 SiO₂ 71.1 70.4 69.8 69.7 68.0 71.5 Al₂O₃ 1.8 2.0 2.94.8 2.5 0.2 MgO 4.4 2.1 3.9 2.1 5.9 4.8 CaO 9.0 11.2 7.8 8.9 8.1 7.2Na₂O 12.6 12.9 14.6 13.2 14.1 15.1 K₂O 0.8 1.1 0.7 0.9 0.9 0.9 SO₃ 0.230.22 0.28 0.09 0.12 0.14 T—Fe₂O₃ 0.019 0.019 0.018 0.018 0.016 0.016TiO₂ 0.04 0.03 0.03 0.04 0.03 0.03 CeO₂ 0 0 0 0 0 0 MnO₂ 0 0 0 0 0 0Total 100.0 100.0 100.0 100.0 100.0 100.0 ZnO 0.010 0.010 0.015 0.0150.020 0.020 FeO 0.005 0.007 0.006 0.005 0.004 0.006 FeO ratio (%) 26 3733 28 25 38 Visible light 91.4 90.8 91.1 91.4 91.5 90.9 transmittance(%) Solar radiation 90.3 89.1 89.8 90.3 90.7 89.5 transmittance (%)Dominant 558 552 553 557 562 552 wavelength (nm) Excitation 0.19 0.180.18 0.19 0.19 0.17 purity (%) Fluorescence 0 1 2 0 0 1 intensity ratioWater 0.59 0.80 0.50 0.15 0.76 1.69 resistance (mg)

TABLE 6 Example 7 8 9 10 11 12 SiO₂ 71.7 71.7 71.6 71.6 71.5 71.5 Al₂O₃1.7 1.7 1.7 1.7 1.7 1.7 MgO 4.2 4.2 4.2 4.2 4.2 4.2 CaO 8.5 8.5 8.5 8.58.5 8.5 Na₂O 13.0 13.0 13.0 13.0 13.0 13.0 K₂O 0.7 0.7 0.7 0.7 0.7 0.7SO₃ 0.12 0.12 0.12 0.12 0.12 0.12 T—Fe₂O₃ 0.015 0.015 0.015 0.015 0.0150.015 TiO₂ 0.02 0.02 0.02 0.02 0.02 0.02 CeO₂ 0 0.04 0.06 0.10 0.14 0.20MnO₂ 0 0 0 0 0 0 Total 100.0 100.0 100.0 100.0 100.0 100.0 ZnO 0.0400.040 0.040 0.040 0.040 0.040 FeO 0.004 0.003 0.003 0.002 0.002 0.001FeO ratio (%) 27 20 20 13 13 7 Visible light 91.2 91.6 91.6 91.7 91.691.6 transmittance (%) Solar radiation 90.0 90.7 90.6 91.0 91.0 91.3transmittance (%) Dominant 554 565 565 570 571 573 wavelength (nm)Excitation 0.19 0.20 0.20 0.20 0.24 0.30 purity (%) Fluorescence 2 21 3128 16 11 intensity ratio Water 0.58 0.58 0.58 0.58 0.59 0.59 resistance(mg)

TABLE 7 13 14 15 16 17 18 SiO₂ 71.0 71.7 71.6 72.0 71.1 71.1 Al₂O₃ 1.41.7 1.7 1.7 1.8 1.5 MgO 4.3 4.0 4.2 4.2 4.4 6.2 CaO 8.6 8.5 8.5 8.5 9.08.7 Na₂O 13.5 13.0 13.0 12.5 12.6 11.1 K₂O 0.7 0.7 0.7 0.7 0.7 1.0 SO₃0.22 0.23 0.20 0.21 0.23 0.23 T—Fe₂O₃ 0.019 0.019 0.011 0.011 0.0130.013 TiO₂ 0.03 0.03 0.04 0.04 0.04 0.04 CeO₂ 0.22 0.10 0.05 0.06 0.100.10 MnO₂ 0 0.06 0 0.08 0 0 Total 100.0 100.0 100.0 100.0 100.0 100.0ZnO 0.020 0.020 0.050 0.050 0.040 0.40 FeO 0.001 0.002 0.002 0.001 0.0020.002 FeO ratio (%) 5 11 18 9 15 15 Visible light 91.6 91.6 91.7 91.891.7 91.7 transmittance (%) Solar radiation 91.2 91.0 91.0 91.3 90.990.9 transmittance (%) Dominant 573 570 567 570 568 568 wavelength (nm)Excitation 0.31 0.23 0.20 0.21 0.20 0.20 purity (%) Fluorescence 9 26 2727 28 28 intensity ratio Water 0.79 0.57 0.52 0.44 0.53 0.44 resistance(mg)

As discussed in the foregoing description, according to the presentinvention, 0.006 to 0.20 wt. % of zinc oxide is contained in asoda-lime-silica glass containing total iron oxide in terms of Fe₂O₃ inan amount of less than 0.02 wt. %, and thus a sufficient effect ofreducing or eliminating the formation of NiS particles can be attained,thereby allowing an improved quality glass product to be obtained.

Furthermore, the addition of the zinc oxide has almost no influence onvisible light transmittance and ultraviolet transmittance, and also hasno influence on physical property values of the glass in terms of acoloring property, viscosity, expansion or the like. Thus, particularly,the general glass quality can be maintained, while securing hightransmittance, thereby achieving a substantial advantage from apractical viewpoint.

Furthermore, the present invention allows manufacturing of glassproducts containing almost no NiS. Thus, also in a process ofmanufacturing a tempered glass, a heating (soaking) process for removingglasses containing NiS can be omitted after a quench tempering process,thereby allowing a manufacturing cost to be reduced. Further, a rate ofglass breakage caused in soaking can be lowered, thereby allowingproduct yield to be improved.

The invention may be embodied in other forms without departing from thespirit or essential characteristics thereof. The embodiments disclosedin this application are to be considered in all respects as illustrativeand not limiting. The scope of the invention is indicated by theappended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

1. A method of manufacturing a high transmittance glass sheet formed ofa soda-line-silica glass composition comprising in wt. % less than0.020% of total iron oxide in terms of Fe₂O₃ and 10% to 18% of Na₂O,wherein the glass further comprises: adding a zinc compound to a glassraw material so that a content of zinc oxide in the high transmittanceglass sheet is 0.006 to 2.0 wt. %; melting the glass raw material; andforming the high transmittance glass sheet, wherein the zinc compound isat least one selected from zinc nitrate and zinc sulfate.
 2. The methodaccording to claim 1, further comprising tempering the hightransmittance glass sheet.
 3. A method of suppressing formation ofnickel sulfide in a high transmittance glass sheet, the hightransmittance glass sheet having a solar radiation transmittance of87.5% or higher and/or a visible light transmittance of 90.0% or higheron a basis of a 4.0 mm thick glass sheet, the method comprising:preparing a glass raw material so that a content of total iron oxide interms of Fe₂O₃ in the high transmittance glass sheet is less than 0.020wt %, a content of zinc oxide in the high transmittance glass sheet is0.006 to 2.0 wt % and a content of sodium oxide in the hightransmittance glass sheet is 10 to 18 wt %; and melting the glass rawmaterial.